Nano-scale diamond (hereinafter referred to as “nanodiamond”) has a large number of excellent properties such as a high hardness and an extremely low coefficient of friction, and therefore, it has been already utilized in various fields and its development of application has been investigated as an extremely promising new material.
It is known that nanodiamond can be, for example, synthesized by utilizing a detonation reaction of a high explosive. This synthesis method is one named a detonation method, in which a detonation is performed with only a raw material substance containing an aromatic compound having 3 or more nitro groups as a carbon source (hereinafter referred to as “low explosive raw material”), and a carbon atom decomposed and liberated, by the detonation reaction, from a molecule constituting the low explosive raw material is formed as diamond at high temperature and high pressure during the detonation (see, for example, NPL 1).
A production of nanodiamond by the detonation method has hitherto been performed in, for example, the East European countries inclusive of Russia and the Ukraine, the United States of America, China, and the like. In these countries, since a military waste low explosive is inexpensively available as the low explosive raw material that is a carbon source, trinitrotoluene (TNT), a high explosive mixture of TNT with hexogen (RDX: trimethylenetrinitramine) or octogen (HMX: cyclotetramethylenetetranitramine), and the like have been used. In the present invention, the high explosive refers to a material capable of performing the detonation reaction, and the low explosive raw material and the non-low explosive raw material are included in the high explosive. In addition, an explosive substance refers to a substance capable of causing an abrupt combustion reaction and includes a substance that is solid at normal temperature and normal pressure and a substance that is liquid at normal temperature and normal pressure; however, the term refers to a solid explosive substance that does not have fluidity at normal temperature and normal pressure unless otherwise indicated in the present specification.
It is anticipated that the demanded amount of the nanodiamond will increase more and more in the future following the development of its application. However, as for the production using a military waste low explosive, there is a limit in the production volume. Therefore, there is a possibility that the supply will be short in the international market in the future. Then the domestic production is expected but, according to an advance evaluation made by the present inventors, it has become clear that, in general, the above-described low explosive raw material is expensive and hence, the production costs become high, leading to unprofitability in economy.
Now, the nanodiamond produced by the detonation method contains carbon impurities mainly composed of nano-scale graphite carbon (hereinafter referred to as “nanographite”) that is a carbon fraction not having a diamond structure.
Hitherto, it has been considered that the carbon impurities are an undesirable existence in utilizing excellent properties of the nanodiamond. Therefore, in conventional techniques, it has been focused to prepare the nanodiamond by removing carbon impurities such as nanographite, as far as possible through various purification methods or chemical treatments (see, for example, PTLs 1 and 2). However, the nanographite has different physical properties such as low hardness and high electroconductivity as compared with the nanodiamond, and in addition, it has such characteristic features that it is able to be bound to a large amount of heterogeneous atom other than carbon, or a functional group, so that it can be given a new function. Therefore, attention is paid to the nanographite as a promise novel material capable of being given various properties through single use thereof or as a mixture with nanodiamond.